The present invention relates to automatic frequency control (AFC) circuits and, more particularly, to a high gain phase locked loop (PLL) AFC system suited to be utilized in combination with a high performance television receiver.
In a typical television receiver AFC system the intermediate frequency (IF) signal is first limited to remove amplitude modulation therefrom and then supplied to a frequency discriminator circuit. A tuned circuit and a multiplier are required to convert a frequency deviation of the IF signal from its nominal frequency into a change in voltage at the output of the multiplier. This voltage is then applied to the local oscillator (LO) in the tuner section of the television receiver to, in turn, cause a resultant change in the IF signal as understood. The tuned circuit and multiplier form a phase detector. Thus, a change in the IF signal from the nominal center frequency of the tuned circuit produces a change in phase from the initial ninety degrees between the two inputs to the double balanced multiplier. This change in phase is therefore the reason a change in the drive voltage to the LO is produced.
The typical AFC system described above has several serious disadvantages associated therewith. First, the tuned circuit has to be adjusted and is in fact a source of radiation that can cause instability in the IF signal detection. Secondly, due to the characteristics of the IF bandpass filter, the AFC loop characteristics are asymmetrical. One of the effects of this asymmetry is that system noise will add a direct current component to any direct current offsets in the loop and, hence, is one of the factors limiting the direct current loop gain. Low loop gain in turn leads to large frequency errors at the input tuner which limits system acquisition which is highly undesirable. Finally, the coil of the tuned circuit is expensive and must be adjusted which is also undesirable.
Hence, a need exists for an improved AFC system having high gain loop characteristics which eliminates the need for a separate AFC tuned circuit